This article reports a study examining the effects of an aggressivity prevention programme implemented in a special education school for adolescents with severe behavioural disorders. The results of the evaluation process carried out after three years of programme activity show a reduction of aggressive behaviours, and an improvement of the students’ behavioural self-control. The quality of the teacher- student relations, the importance of collaboration between intervention partners, as well as the school team’s level of programme appropriation are discussed in the article. Interesting findings concerning the development of mutual helping skills between students in schools showing a high degree of aggressivity are finally presented.